Short channel effects are getting more significant as planar semiconductor devices are increasingly being scaled down. To this end, three-dimensional (3D) semiconductor devices, such as Fin Field Effect Transistors (FinFETs), have been proposed. Generally, a FinFET includes a fin formed vertically on a substrate and a gate stack intersecting the fin. In addition, an isolation layer is formed on the substrate to isolate the gate stack from the substrate. As such, the fin has its bottom surrounded by the isolation layer. Therefore, it is difficult for the gate to effectively control the bottom of the fin. As a result, a leakage current tends to occur between a source and a drain via the bottom of the fin.